Scroll-type fluid displacement mechanisms are generally known. The general operation of such mechanisms is disclosed, for example, in U.S. Pat. No. 801,182 issued to Creux. Typically, these devices include a pair of scroll elements, each of which has an end plate and an involute wrap projecting from the end plate. The mechanism is usually structured such that the scrolls extend toward one another and are angularly and radially offset so that the involute scrolls interfit to make a plurality of line contacts between their curved surfaces to thereby seal off and define at least one pair of fluid pockets. Typically, one scroll element remains fixed, while the other scroll element orbits, without rotating, about the center of the fixed scroll element. The orbiting motion may be produced by a crank element rotated by a power supply. The relative orbital motion of the two scrolls shifts the line contacts along the involute curved surfaces of the scrolls and, as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases as a function of the direction of orbital motion, a scroll-type fluid displacement mechanism may be used to compress, expand or pump fluids.
As discussed, the scrolls of these devices are characterized by one or more involute curves. As shown in FIG. 1, for example, involute curve 10 defines the shape of a conventional form of a first scroll. Involute curve 10 of FIG. 1 can be described by the equation l =r.sub.g.multidot..phi., where "l " is the length of a line segment 30 tangential to base circle 20 and extending from the tangent point 31 to a point 15 on curve 10, "r.sub.g " is the radius of base circle 20, and ".phi." is the angular displacement from axis "x" (which passes through the starting point 11 of curve 10 and the center of the base circle) to a line extending outwardly from and perpendicular to line segment 30.
To create a pair of scroll elements capable of forming the desired fluid pockets, a second scroll is provided defined by involute curve 12, which starts at starting point 13 on the same base circle 20, but 180.degree. offset from the first curve as shown in FIG. 2. As shown in FIG. 3, inner and outer walls 3 and 4 of the first scroll and inner and outer walls 5 and 6 of the second scroll are then defined by shifting the involute curves 10 and 12 about the center of base circle 20 by angles .phi..sub.1 and .phi..sub.2.
When the scrolls are designed by involute curves being separated by a distance R.sub.or (FIG. 3), then the walls of the scrolls may be brought into contact with each other to form a plurality of fluid pockets, as shown in FIG. 4. As one scroll moves in an orbiting path with respect to the other, the fluid pockets migrate toward the center of the scroll elements and their contents are eventually expelled at increased pressure and reduced volume from one or more ports near the center of the scroll elements (FIG. 5).
As shown in FIG. 6, this scroll-type apparatus has the disadvantage that a large area is unused near the outer periphery of the device (FIG. 6). As shown in FIG. 7, the center of the scroll pair may be offset from the center of the crank element in order to reduce this wasted space. However, this design causes a variation in the gas reaction forces experienced by an anti-rotation coupling, which is used to prevent rotation of the orbiting scroll. As depicted in FIG. 8A, the moment on the anti-rotation coupling becomes M.sub.c =-M.sub.g -(e cos .theta..times.F.sub.d), where Mg is the moment about the geometric center 41 of an orbiting scroll 42 due to pumping loads, .theta. is the angular offset of a crankshaft throw 40 from a line through the centers of the orbiting scroll and a scroll drive bearing 43, e is the offset of the scroll pair geometric center 41 from the center of scroll drive bearing 43, and where F.sub.d is the force applied to the scroll drive bearing 43 to cause the scroll to describe its orbital motion. It can be seen that under certain circumstances the moment on the anti-rotation coupling can reverse, thereby causing noise and durability problems. As shown in FIG. 8B, for example, .theta.=180.degree., hence cos .theta.=-1. Therefore, M.sub.c =-M.sub.g +(e.times.F.sub.d). If e.times.F.sub.d &gt;M.sub.g, then M.sub.c can sometimes be positive and sometimes be negative.
Another problem in prior art scroll design concerns formation of the central portions of the scrolls. For example, when involute curves are used to define the surfaces of the scroll walls, these involute curves stop at the surface of the involute base circle as shown in FIG. 3. Therefore, in order to complete the center portion of the scroll wall, it has been necessary to deviate from the true involute curve. Wall segments A1, A2 and A3 in FIG. 9 show typical non-involute forms used at the center of a scroll defined by an involute curve.
One solution to the aforementioned problems might be to use a spiral curve (FIG. 10) to define the scroll. The spiral curve passes through the origin and does not require a base circle to be generated. FIG. 10 shows a typical spiral curve with a constant pitch, which can be described by the equation r=c.beta., where "r" is the distance from the origin to any point on the spiral curve, ".beta." is the angular offset from the "x" axis (which lies along the initial path of the spiral curve), and "c" is a constant. Since the spiral curve passes through the origin, as shown in FIG. 10, if a spiral curve could be used to define the shape of a scroll, it would not be necessary to deviate from the true curve near the center portion of the scroll. However, proper contact cannot be achieved when using spirals to define the surfaces of two scroll walls. For example, in FIG. 10, with r=c.beta., the slope of the spiral curve at .beta. is dy/dx(.beta.)=(.beta. cos .beta.+sin .beta.)/(-.beta.sin .beta.+cos .beta.). Thus, for a spiral curve, dy/dx(.beta.) is not equal to dy/dx(.beta.-180.degree.). By contrast, in the case of an involute curve, dy/dx(.phi.)=tan (.phi.). Thus, dy/dx(.phi.)=dy/dx(.phi.-180.degree.). This structure permits the walls of the involute scrolls to come into contact without interference. Therefore, prior art scroll mechanisms have required that involute curves be used to define the scroll wall surfaces even though the central regions of the scrolls must deviate from the true form of the involute curve.
As shown in FIG. 11, another design technique has been employed, which involves using a hybrid spiral curve to generate the form of the scrolls. This method has the benefit that the available space in the apparatus is used more efficiently than if the scrolls are of the conventional circular involute form. However, the scrolls must of necessity be asymmetric which introduces unbalanced gas reaction forces into the mechanism.
In the operation of conventional scroll-type mechanisms, one of the most common causes of failure is breakage of the scroll walls due to the large cantilevered forces generated by the pressure differential across the scrolls. Current scroll designs are based on the assumption of constant wall thickness. Even the hybrid scroll concept described above relies on one wall being of constant thickness in order to make the design of the scroll pair manageable, as this design concept requires the use of curve fitting techniques in order to derive the shape of the curves. However, the gas force on the wall increases continuously toward the center of the scroll. As a result of using a constant wall thickness, the outer portion of the wall is overly thick for the low gas forces achieved, while the inner tip must be artificially increased in thickness. In order to thicken the inner tips of the scroll walls, their shapes must deviate greatly from that derived from the defining equation. Also, the excessive thickness of the outer portions of the wall means that wall material is taking up space which could be used for fluid compression. Further, the necessity to increase the thickness of the inner tips causes discontinuities in the gas compression process.
These are just examples of problems associated with prior art scroll design. It will be recognized by those having ordinary skill in the pertinent art that other problems and disadvantages result from the design of known scroll-type mechanisms.